STM study of organic molecules on a hexagonal SiC surface Tamara Ovramenko PhD supervisors: Andrew Mayne Gérald Dujardin Groupe de Nanosciences Moléculaires Bât. 210, Université Paris-Sud, Orsay, France

Objectives Molecular SiC “MOLSIC” To functionalize SiC surface with organic molecules We study interaction of different kinds of molecules with wide band gap 6H-SiC (0001) 3  3 surface

Ultra High Vacuum Scanning Tunneling Microscope STM (P  3  to 8  Torr) STM head

6H-SiC (0001) 3  3 G. Baffou, A.J. Mayne et al Phys. Rev. B 77, (2008) Surface states diagram Molecular SiC “MOLSIC” Si adatom, with non compensated dangling bonds Surface state inside band gap S2=-1.5eV, 0.03 G  S1=-0.5eV, 1.0 G  U1=0.5eV, 1.0 G  Top: top view of unit cell Bottom: bulk cross-section Dangling bonds minimisation

STM images of 6H-SiC (0001) 3  3 a) c) b) Molecular SiC “MOLSIC” Fullerene C 60 C 60 mean diameter 0.7nm band gap size 1.7eV well studied on other surfaces fluorescent STM images of C 60 on 6H-SiC (0001) 3  3

What are the adsorption positions of C 60 on SiC? Si-3 Between three adatoms E ads =-0,28 eV Si-1 On top of Si adatom E ads =-0,67 eV Theoretical calculation

Single C 60 molecule adsorption positions Position “2” Chemisorbed: E=-0,21 eV Position “3” Chemisorbed: E=-0,28 eV Top view Position “Top” Chemisorbed: E=-0,67 eV

What are the adsorption positions of C 60 on SiC? Si-3 Between three adatoms E ads =-0,28 eV 20% Si-1 On top of Si adatom E ads =-0,67 eV 40% Experimental resalts Si-2 Between two adatoms E ads =-0,21 eV 40%

Position “Top” Position “2” Position “3”

Statistical analysis and 2D Poisson distribution analysis -Analyze surfaces with 3 different deposition times (10s-60s) -Define molecular coverage corresponding to the each deposition time (1.8%-12.8%) -Determine percentage for each molecular cluster Time, sCoverageM1M1 M2M2 M3M3 M4M4  M5 101/15 ML63%21%16%00 251/8 ML44.6%35.5%11.7%1.7%0 601/4 ML37%22%18%6%17% 2D Poisson distribution used to analyse the distribution of C60 for possible clustering Probability to find k molecules in a particular square We have more empty (k=0) squares than expected 2D Poisson distribution clearly indicates clustering M. Cranney et al, Appl. Phys. A 94, 767(2009) 2D Poisson distribution analysis Do they “like” to form clusters?

Creating a monolayer of C 60 on SiC 2.5V, 1/8ML -4.5V, 1/4ML -4.0V, 1ML Objectives: distribution of C 60 molecular clusters packing of monolayer Monolayer of C 60 on SiC not complete less than one molecule per SiC unit cell no long range packing in monolayer just locally ordered packing

Work in progress and Perspectives Caltrop molecule - is complex fluorescent molecule consisting of four linked PTCDI molecules. Idea: To chemisorb it on the SiC surface with help of 3 PTCDI “legs”, leaving one “leg” free for luminescence. Results: 1.Deposited Caltrop on SiC. Complex: images of molecule convoluted with tip. Fragile: decomposed into fragments. 2. STM images of caltrop deposited on Si (100) were obtained. Perspective: to deposit new synthetized Caltrop with reinforced center on Si (100) and SiC Terphenyl acid molecule Idea: To chemisorb molecule next-nearest neighbor Si adatoms creating a “molecular bridge” between two adatoms. Results: 1.Deposited TAM on Si(100) dissociate into small components 2.Si surface can not be recuperated after deposition of Terphenyl

SiC nanowire flowers Ghim Wei Ho et al, Nanotechnology 15, (2004) Thank you for your attention

RT STM image of C 60 on SiC at +2.5V Internal structure and orientation G. Schull et al, Phys. Rev. Lett. 99, (2007) N. Néel et al, Phys. Rev. B 77, (2008) LT STM image of C 60 on Cu (100) at 8KLT STM image of C 60 on Au (111) (one lobe) (two lobe) (three lobe)

C 60 position analyses procedure

1. Cut a piece of clean SiC surface 2. Overlay on C 60 molecules 3. Make piece 30-40% transparent and match to Si adatoms of original picture with Si adatoms of transparent piece.